Amplifier control circuits



Feb. 17, 1942. OOSTERHUIS AMPLIFIER CONTROL CIRCUITS I Filed March'6, 1940 270 NEXT GALVANOMETER 70 s/a/wu RECT/F/E IQ SIGNAL SOUR INVENTOR W W mi 0 3mm 07 My EB Patented Feb. 17, 1942 AMPLIFIER CONTROL omcurrs Ekko oosterhuis, Eindhoven, Netherlands, 'assignor to Radio Corporation of America, a corporation of Delaware Application March 6, 1940, Serial No. 322,461 g In the Netherlands July 11, 1939 3 Claims. (Cl. 179-171) This invention relates to a circuit arrangement for the transmission of electrical oscillations .which includes at least one electron discharge tube and means for controlling the amplification thereof by a control voltage which acts upon the tube.

In such circuit arrangements a control voltage is usually applied to the input grid of the tube and by which voltage the working slope of the grid voltage-anode current characteristic is acted upon in a desired manner. insuch control circuits that, simultaneously with the desired amplification variation, there occur considerable variations of the working capacity between the various connecting points, more particularly between the grid and the cathode of the tube. Such'variations give rise to detuning of the connected oscillatory circuits if the order of magnitude of capacity of the oscillatory circult can be compared with that of the capacityvariations of the tube. With the control applied the input capacity of the tube between the control grid and the cathode may in many cases vary by 1.5 mmf, With a capacity of the oscillatory circuit,of 75 mmf., the value of the detuning involved thereby may amount to-about 1% of the In the circuit arrangement shown in Fig. 1 the signal oscillations to be amplified, which are applied to the terminals of an oscillatory circuit l,

are supplied through a condenser 2 to the signal grid 3 of an amplifying tube 4 provided with variable amplification. Amplification control of tube 4 is effected "by supplying a control voltage to the signal grid 3 through a resistance 5. The amplified oscillations are supplied to an in- It has been found maining part of the circuit arrangement (not ductance 6 disposed in the anode circuit of tube 4, and with which inductanceis coupled the rerepresented in the drawing). The control voltage required for controlling the amplification of tube 4 is taken from a resistance 8 in the cathode lead of tube 4. The resistance 8 is furnished with a sliding contact I and is bridged by a condenser 9 having a negligible impedance for alternating currents of the frequency to be amplified.

Upon controlling the bias of the signal grid 3 of tube 4 the space charge in this tube varies which involves at the same time a variation of the input capacity of tube 4. In order to cominitial resonance frequency of the tuned circuit.

7 When the connected tuned circuits have a small damping and a corresponding high selectivity,

such detuning manifests itself in a very annoying manner.

The present inventionpermits the said variations of the working capacity to be avoided in controlling the amplification. According to the invention the variations in capacity'caused by this end a fixed resistance is preferably con- 5 nected in series with the variable condenser.

The, invention will be more fully explained by reference to the accompanying drawing wherein, Fig. 1 represents a circuit arrangement in -which amplification is controlled manually and the correcting condenser is operated at the same time, 1 1

Fig. 2 shows a circuit arrangement wherein control of amplification and'the correcting condenser is efiected automatically;

pensate for this variable input capacity a variable condenser I0 is connected between the signal grid 3 of tube 4 and the grounded electrode of condenser 9. This variable condenser I0 is mechanically connected to the sliding contact 1 of *resistance 8, The mechanical coupling is indicated by a dotted line H. When controlling the amplification by displacing the sliding contact I the condenser I0 is at the same time controlled in such manner that the input capacity of tube 4 remains at least substantially constant.

In the circuit arrangement shown inFig. 2

corresponding parts of Fig. 1 bear the same reference numerals. In Fig. 2 the amplification of tube 4 is not controlled manually but automatically. For this purpose the signal grid 3 is connected through the resistance 5 to a lead 12 which is connected to a signal rectifier for automatic volume control (not represented). The

letters AVC designate the connection to a usual automatic volume control rectifier. During am-- ,plification control the input capacity and at the same time the anode direct current of tube 4 vary. The anode circuit includes a galvanometer device I3 which is mechanically coupled with the variable condenser ID for compensation of the input capacity, and which mechanical coupling is indicated by a dotted line I4. A

fixed resistance I5 is connected in series with the variable condenser ID. This resistance l5 serves to make up for the damping which varies with the amplification exerted by theinput impedance of tube 4 on the oscillatory circuit I.

When the negative bias' of the signal grid 3 increases the input capacity of tube 4 decreases,

and at the' same time the damping exerted on the circuit decreases. In order to make up for the decreasinginput capacity of tube 4 the variable condenser I is controlled by the galvanom-- eter device in such a manner that the capacity increases. As a result thereof the resistance I5 connected in series with the variable condenser l0 exerts a damping effect on the oscillatory circuit, and which damping increases with increasing negative bias and makes up for the damping which is exerted by tube 4 on the oscillatory circuit I and decreases with increasing by the capacity existing between said control electrode and cathode varies, a device connected between said input terminals providing a compensating 'capacity'thereacross, and means conjointly operative with changes in said potential difference for varying the magnitude of said compensating capacity in a sense to prevent said first capacity from being efiective across said input terminalsand a resistor in series with said device whose damping efiect between said input terminals increases wtih increase of the compensating capacity magnitude.

value with gain variation, from aflecting thev resonant frequency of said input circuit, said improvement comprising an adjustable capacitance device operatively associated with said input circuit to oppose the effect of said component, and means for controlling said device in response to operation of said bias varying means .and in a sense to compensate for variations in the magnitude of said component said device having a resistor of predetermined value in series therewith across said input circuit, and said resistor compensating for variations in the resistive component of said impedance.

3. A signal amplifier of the type having a resonant input circuit, said amplifier comprising a tube having at least a signal grid, cathode and output electrode, means for varying the bias of said grid over a range such that the inherent tube grid to cathode impedance veriation afiects the input circuit response characteristic, means connected in shunt across said input circuit for providing a compensatory impedance thereacross, and means for control of said shunt means in response to said varying means, said inherent impedance having a capacity component and a resistive component, and said shunt means including an adjustable capacity in series with a resistor whose damping effect on said input circuit increases with increase of magnitude of said adjustable capacity.

EKKO OOSTERHUIS. 

